This proposal is an epidemiologic study of HTLV-III infection within a population of intravenous drug abusers (IVDAs) in treatment. This study proposes: (1) to calculate the prevalence of antibody to HTLV-III among a population of IVDAS in treatment. (2) to establish the profile of IVDAs in treatment who are at high risk for HTLV-III, (3) to calculate the incidence rate of seroconversion among the IVDAs in treatment, (4) to identify risk behaviors that are associated with IVDAs becoming infected with HTLV-III, and (5) to assess the impact that being screened (seropositive or seronegative) has upon subsequent risk behaviors of IVDAs. This study designed as a prospective cohort investigation of antibody to HTLV-lII among IVDAs in treatment. Within this cohort, a nested case-control study will be conducted, cases being incident HTLV-III seropositive individuals and controls being seronegative IVDAs. There are four phases: 1. a cohort of IVDAs who are in a drug treatment program will be enrolled, screened for antibody to HTLV- III, and interviewed about relevant risk factors. These data will be analyzed to determine the prevalence of antibody to HTLV-III and to develop a profile of high risk IVDAs who test seronegative at baseline will be entered into phase 2 repeat testing for seroconversion will be performed every 6 months (for a period up to 3 years) to determine the proportion who become seropositive, as well as to identify individuals for the case-control study. In phase 3, each IVDA who becomes seropositive after the initial screen will be entered into a case control study designed to identify risk factors for HTLV-III positivity. As seroincident cases are identified, seronegative controls will be selected and interviewed (blindly). Standard epidemiologic procedures will be employed to analyze these data. Phase 4 is a follow-up of 150 individuals who were seroprevalent at baseline and 150 seronegative IVDAs to determine how knowledge of being screened positive while in treatment changes drug use and other behavior after IvDAs leave treatment.